The present invention relates to nitride-based acoustic wave devices. Acoustic wave devices form a class of electronic devices that process signals that exist as acoustic (i.e. sound or compression) waves traveling in piezoelectric crystals. Piezoelectric crystals are characterized by the fact that when the material is mechanically stressed (i.e. compressed or placed under tension), an associated electric field is induced. Likewise, when an electric field is applied to a piezoelectric crystal, the material becomes mechanically stressed in a predetermined manner. It is possible to exploit these characteristics to perform many different functions with a piezoelectric crystal.
For example, piezoelectric microphones convert acoustic waves traveling though air into electronic signals. Piezoelectric speakers and buzzers perform the opposite function. Piezoelectric sensors detect changes in pressure, temperature, torque, humidity and/or a wide range of other phenomena.
Common piezoelectric materials include quartz (SiO2), zinc oxide (ZnO), barium titanate (BaTiO3), lithium tantalate (LiTaO3) and lithium niobate (LiNbO3). However, other materials, most notably silicon carbide (SiC) and the Group III-nitride materials such as aluminum nitride (AlN) and Gallium Nitride (GaN) are piezoelectric and may be used to form acoustic wave devices.
When a time-varying electric field is applied to a portion of a piezoelectric crystal, the applied electric field induces an acoustic wave that propagates through the crystal. Acoustic waves may travel through a piezoelectric material in a number of modes. For example, acoustic waves may travel through the body of the material—so-called “bulk” waves—or on the surface of the material. Waves that travel along the surface of the piezoelectric material are generally referred to as surface acoustic waves (or SAWs), and devices that process surface acoustic waves are referred to as surface acoustic wave devices, or SAW devices.
A simple surface acoustic wave device comprises a piezoelectric crystal or a thin film of piezoelectric material on a substrate. Interdigitated metal stripes on the surface of the crystal form transmitting and receiving electrodes. The metal electrodes convert electrical energy into mechanical stress in the crystal and vice versa. Hence, the interdigital electrodes formed on a piezoelectric material are referred to as interdigital transducers, or IDTs.
A simple surface acoustic wave device is illustrated in perspective in FIG. 10. The SAW device comprises a piezoelectric film 2 formed on a substrate 1. A metal (usually aluminum) is deposited on the film and patterned using standard photolithographic or liftoff techniques to form the input IDT 3 and the output IDT 4. The thickness of the piezoelectric film is generally on the order of one SAW wavelength.
In operation, an electric signal may be applied to the input IDT 3. The input signal causes a surface acoustic wave to be induced in the piezoelectric film 2 and propagate along the surface of the film 2 towards the output IDT 4. The shape of the generated wave depends on the electric signal applied to the input IDT, the design and orientation of the IDT fingers, and the piezoelectric material used. When the wave reaches the output IDT 4, a voltage is induced across the fingers of the IDT 4 which is then output from the device. The shape of the output wave is affected by the design of the output IDT 4.
FIG. 11 illustrates some design parameters for IDTs. The finger period D determines the wavelength λ of the SAW generated by the IDT. The linewidth L and space S of the fingers are generally equal to λ/4. The number of fingers determines the coupling efficiency of the IDT, and the width W of the overlap of fingers affects the frequency response of the finger pair. By changing the overlap of finger pairs within an IDT, various filter functions can be realized.
Surface acoustic wave devices have many different applications in digital and analog electronics. For example, surface acoustic wave devices may be used as bandpass or bandstop filters, duplexers, delay lines, resonators and/or impedance elements among other things. They may also be used to perform digital functions such as convolution, correlation, pulse compression and/or digital filtering (for example in spread-spectrum communication systems) depending on the design of the device, and in particular depending on the layout of the interdigital transducers. The design and fabrication of surface acoustic wave devices are described in Chapter 66 of K. Ng, Complete Guide to Semiconductor Devices, McGraw Hill (1995).
The velocity of surface acoustic waves in a device depends on the material from which the device is constructed and the mode of propagation of the SAW. For example, the propagation velocity (also called the SAW velocity) of first order Rayleigh-mode acoustic waves in GaN is around 3600 m/s, while the corresponding SAW velocity in AlN is about 5800 m/s, and over 6800 m/s in SiC. For RF devices, the SAW velocity determines the bandwidth of signals that can be processed by the device. The fundamental operating frequency (f0) of a SAW device is given by the formula:
      f    0    =      v    λ  where ν is the SAW velocity and λ is the wavelength. As discussed above, the wavelength of the device is determined by the finger period of the IDT. The width and spacing of IDT fingers (and thus the finger period) is limited by the resolution of photolithographic techniques. Thus, for a given finger period, increasing the SAW velocity increases the fundamental operating frequency of the device. Stated differently, having a higher SAW velocity permits a device to process higher-frequency signals for a given device geometry. Accordingly, the Group III-nitrides and SiC may be desirable piezoelectric materials for the fabrication of SAW devices.
Group III-nitrides and silicon carbide may also be desirable materials for the fabrication of high power, high temperature and/or high frequency devices. These wide bandgap materials have high electric field breakdown strengths and high electron saturation velocities as compared to other semiconductor materials such as gallium arsenide and silicon.
Electrical circuits requiring high power handling capability (>20 watts) while operating at high frequencies such as radio frequencies, including for example, S-band (2–4 GHz) and X-band (8–12 GHz), have in recent years become more prevalent. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors that are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads. Previously, bipolar transistors and power metal-oxide semiconductor field effect transistors (MOSFETs) have been used for high power applications but the power handling capability of such devices may be limited at higher operating frequencies. Junction field-effect transistors (JFETs) were commonly used for high frequency applications but the power handling capability of previously known JFETs may also be limited.
Recently, metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction may be preferable for high frequency applications because only majority carriers carry current. The MESFET design may be preferred over current MOSFET designs because the reduced gate capacitance permits faster switching times of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET may make the MESFET more desirable for high frequency applications.
In addition to the type of structure, and perhaps more fundamentally, the characteristics of the semiconductor material from which a transistor is formed also affects the operating parameters. Of the characteristics that affect a transistor's operating parameters, the electron mobility, saturated electron drift velocity, electric breakdown field and thermal conductivity may have the greatest effect on a transistor's high frequency and high power characteristics.
Electron mobility is the measurement of how rapidly an electron is accelerated to its saturated velocity in the presence of an electric field. In the past, semiconductor materials which have a high electron mobility were preferred because more current could be developed with a lesser field, resulting in faster response times when a field is applied. Saturated electron drift velocity is the maximum velocity that an electron can obtain in the semiconductor material. Materials with higher saturated electron drift velocities are preferred for high frequency applications because the higher velocity may translate to shorter times from source to drain.
Electric breakdown field is the field strength at which breakdown of the Schottky junction and the current through the gate of the device suddenly increases. A high electric breakdown field material may be preferred for high power, high frequency transistors because larger electric fields generally can be supported by a given dimension of material. Larger electric fields may allow for faster transients as the electrons can be accelerated more quickly by larger electric fields than by smaller.
Thermal conductivity is the ability of the semiconductor material to dissipate heat. In typical operations, all transistors generate heat. In turn, high power and high frequency transistors usually generate larger amounts of heat than small signal transistors. As the temperature of the semiconductor material increases, the junction leakage currents generally increase and the current through the field effect transistor generally decreases due to a decrease in carrier mobility with an increase in temperature. Therefore, if the heat is dissipated from the semiconductor, the material may remain at a lower temperature and may be capable of carrying larger currents with lower leakage currents.
In the past, high frequency MESFETs have been manufactured of n-type III–V compounds, such as gallium arsenide (GaAs) because of their high electron mobilities. Although these devices provided increased operating frequencies and moderately increased power handling capability, the relatively low breakdown voltage and the lower thermal conductivity of these materials have limited their usefulness in high power applications.
Silicon carbide (SiC) has been known for many years to have excellent physical and electronic properties which should theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon (Si) or GaAs. The high electric breakdown field of about 4×106 V/cm, high saturated electron drift velocity of about 2.0×107 cm/sec and high thermal conductivity of about 4.9 W/cm-K indicate that SiC would be suitable for high frequency, high power applications.
SiC-based MESFET structures and their fabrication are described in U.S. Pat. No. 5,270,554 to Palmour et al. and U.S. Pat. No. 5,925,895 to Sriram et al., both of which are incorporated herein by reference as if fully set forth herein. SiC MESFET structures and fabrication are also described in U.S. application Ser. No. 09/567,717 filed May 10, 2000 by Allen, et al., the disclosure of which is incorporated herein by reference as if fully set forth herein.
In the nitride arena, a device of particular interest for high power and/or high frequency applications is the high electron mobility transistor (HEMT), which is also known as a heterostructure field effect transistor (HFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped, smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 1013 carriers/cm2. Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.
This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity.
In electronic communication systems, it is usually desirable to amplify signals before transmission or after reception. It is also often desirable to filter such signals immediately before or after amplification. In high frequency communication systems, such amplification may be performed efficiently using an amplifier circuit incorporating a SiC MESFET or a Group III-nitride based transistor. Filtering may be efficiently performed using a SAW filter.
In order to minimize the number of circuit elements required to implement a communication system and simplify its design, it is desirable to integrate as many components as possible on a single chip. Attempts to integrate SAW devices with other devices have been made. However, such devices have typically required that the piezoelectric crystal be bonded onto a semiconductor substrate (such as silicon) on which active electronic components are formed.